1. Field of the Invention
This invention relates to a II-VI group compound crystal article and a process for producing the same, particularly to a II-VI group compound monocrystalline article or a II-VI group compound polycrystalline article controlled in grain size prepared by utilizing the nucleation density difference of the deposition materials depending on the kind of the materials for the crystal forming surface and a process for producing the same.
The present invention may be applied for formation of crystal such as monocrystal, polycrystal, etc. to be used for, for example, semiconductor integrated circuit, optical integrated circuit, optical device, etc.
2. Related Background Art
In the prior art, monocrystalline thin films to be used for semiconductor electronic device, optical device, etc. has been formed by epitaxial growth on a monocrystal substrate. For example, on a Si monocrystal substrate (silicon wafer), Si, Ge, GaAs, etc. have been known to be epitaxially grown from liquid phase, gas phase or solid phase, and also on a GaAs monocrystal substrate, monocrystals of GaAs, GaAlAs, etc. have been known to be epitaxially grown. By use of the semiconductor thin film thus formed, semiconductor devices and integrated circuits, emission devices such as semiconductor laser or LED are prepared.
Also, in recent years, researches and developments have been abundantly done about ultra-high speed transistor by use of two-dimensional electron gas, ultra-lattice device utilizing quantum well, etc., and these have been made possible by high precision epitaxial technique such as MBE (molecular beam epitaxy) by use of ultra-high vacuum, MOCVD (organometallic chemical vapor deposition), etc.
In such epitaxial growth on a monocrystal substrate, it is necessary to take matching in lattice constant and coefficient of thermal expansion between the monocrystal material of the substrate and the epitaxial growth layer. If such matching is insufficient, lattice defects will be generated in the epitaxial layer. Also, the elements constituting the substrate may be sometimes diffused into the epitaxial layer.
Thus, it can be understood that the process for forming a monocrystal thin film of the prior art according to epitaxial growth depends greatly on its substrate material. Mathews et al examined the combinations of the substrate materials with the epitaxial growth layers (EPITAXIAL GROWTH, Academic Press, New York, 1975 ed. by J. W. Mathews).
Also, the size of the substrate is presently about 6 inches for Si wafer, and enlargement of GaAs, sapphire substrate is further delayed. In addition, since the production cost of a monocrystal substrate is high, the cost per chip becomes high.
Thus, for forming a monocrystal layer capable of preparing a device of good quality according to the process of the prior art, there has been the problem that the kinds of the substrate material are limited to an extremely narrow scope.
On the other hand, in recent years, research and development have been actively done about three-dimensional integrated circuits for accomplishing high integration and multi-functionality by forming semiconductor elements by lamination in the direction normal to the surface of the substrate, and also research and development about a large area semiconductor device such as a solar battery in which elements are arranged in an array on an inexpensive glass or switching transistor of liquid crystal picture elements, etc. are becoming more active year by year.
What is common in both of these techniques is that the technique to form a semiconductor thin film on an amorphous insulating material substrate and form an electronic element such as transistor, etc. in the semiconductor thin film is required. Among them, it has been particularly desired to have a technique to form a monocrystalline semiconductor layer of high quality on an amorphous insulating material substrate.
Generally speaking, when a thin film is formed on an amorphous insulating substrate such as SiO.sub.2, etc., due to deficiency of long length order of the substrate material, the crystal structure of the deposited film becomes amorphous or polycrystalline. Here, "amorphous film" refers to one with the state in which short length order to the minimum extent on the order of atom may be maintained, but there is no longer length order, while "polycrystalline film" refers to one in which monocrystal grains having no specific crystal orientation are gathered as separated with grain boundaries.
For example, when Si is formed on SiO.sub.2 by the CVD method, if the deposition temperature is about 600.degree. C. or lower, amorphous silicon is formed, while at a temperature higher than that, polycrystalline silicon with grain sizes distributed between some hundreds to some thousands .ANG. is formed. However, the grain size and its distribution will vary greatly depending on the formation method.
Further, a polycrystalline thin film with a large grains size of about micron or millimeter is obtained by melting and solidifying an amorphous or polycrystalline film with an energy beam such as laser, rod-shaped heater, etc. (Single-Crystal silicon on non-single-crystal insulators, Journal of Crystal Growth vol. 63, No. 3, Oct. 1983, edited by G. W. Cullen).
When transistors are formed in thin films of various crystal structures thus formed, and electron mobility is measured from its characteristics, a mobility of ca. 0.1 cm.sup.2 /V.multidot.sec is obtained for amorphous silicon, a mobility of 1 to 10 cm.sup.2 /V.multidot.sec for polycrystalline silicon having a grain size of some hundred .ANG., and a mobility to the same extent as in the case of monocrystal silicon for polycrystalline silicon with a large grain size obtained by melting and solidification.
From these results, it can be understood that there is great difference in electrical characteristics between the element formed in a monocrystal region within the crystal grain and the element formed as crossing over the grain boundary. In other words, the semiconductor deposited film on an amorphous substrate obtained according to the prior art method has an amorphous structure or a polycrystalline structure having grain size distribution, and the semiconductor electronic element prepared in such deposited films is greatly inferior in performance as compared with a semiconductor electronic element prepared in a monocrystal layer. For this reason, uses are limited to simple switching element, solar battery, photoelectric transducing element, etc.
Also, the method for forming a polycrystalline thin film with a large grain size by melting and solidification had the problem that enormous time is required for making grain size larger, because each wafer is scanned with an energy beam to convert an amorphous or polycrystalline thin film to a polycrystalline thin film with a large grain size, whereby bulk productivity is poor and the method is not suited for enlargement of area.
On the other hand, II-VI group compound semiconductors are expected to be a material capable of realizing a new device not realized by Si, such as ultra-high speed device, optical element, etc., but II-VI group compound crystal can be grown only on Si monocrystal substrate or a II-VI group compound monocrystal substrate, which has been a great obstacle in preparation of a device.
As described above, in the crystal growth method of II-VI group compound crystal of the prior art and the crystal formed thereby, three-dimensional integration or area enlargement cannot be easily done, and practical application for a device has been difficult, whereby a crystal such as monocrystal, polycrystal, etc. required for preparing a device having excellent characteristics cannot be formed easily and at low cost.